Stability and Physical Properties of Metastable Ba–Si Clathrates

Abstract

In the present study, we have investigated the stability of different Ba−Si clathrates with pressure and temperature using DFT calculations and studied the stability of type I Ba 8 Si 46 and type IX Ba 24 Si 100 clathrates using high pressure� high temperature synthesis technique, calorimetry, and diffraction experiments. When increasing pressure, the type I Ba 8 Si 46 clathrate and BaSi 6 become more stable. In good qualitative agreement with experiments, the type IX Ba 24 Si 100 clathrate becomes stable at a pressure of 1−2 GPa thanks to the pressure and thermal effect of both electronic and vibrational contributions. One can notice that the presence of Ba in the cages of type IX clathrate increases significantly the stability and the mechanical properties of type IX clathrate. We have determined the P−T existence domain of type IX Ba 24 Si 100 clathrate from ex situ experiments, which was confirmed by in situ synchrotron X-ray experiments. At room pressure and under an oxidizing atmosphere, the type I Ba 8 Si 46 and the type IX Ba 24 Si 100 clathrates are stable up to about 560 °C and up to about 600 °C, respectively. The thermoelectric properties of type IX Ba 24 Si 100 are also reported.